The most conventional date timepieces are described in particular in the work entitled “Les montres compliquées” (A Guide to Complicated Watches) by François Lecoultre and edited by Editions Horlogères in Bienne.
In a calendar mechanism, a ratchet or month star carrying a month cam is driven, at each change in month, by a lever which cooperates with a day-snail and jumps when the steep side of the day-snail is crossed.
The period of one complete revolution of the month cam is usually twelve or forty-eight months.
This month cam includes, on the periphery thereof, notches or surfaces, the radial dimension of which corresponds to the duration of the different months, and which occupies a significant amount of space which is not always compatible with that of the other mechanisms and complications in the timepiece.
The function of a so-called perpetual calendar device is to determine the number of days in the current month, and, more specifically, in the month of February. The perpetual mechanism is an approximate notion: most commercially available mechanisms are simple leap year mechanisms, either using a month cam with 48 notches, or a month cam with twelve positions, where the February position includes a leap year mechanism with a Maltese Cross or similar element, to mention the best known devices.
The design of a perpetual calendar device meets with two difficulties:
how to take account of the specificities of the type of calendar concerned and translate this into the form of a timepiece mechanism, and
how to update such a mechanism in case of stoppage. Updating is often so complex that the timepiece must never be stopped, as is the case of astronomical clocks for buildings. Even in the case of the most basic version of a perpetual Gregorian calendar that simply manages leap years in four-year cycles, any updating is accomplished by a large number of operations, up to 47 manoeuvres to arrive at the right year and the right month, which results in wear of the mechanisms.